The domestic water heater market is dominated by natural gas pulse heaters and electric water heaters which are attractive to consumers because of their relatively low initial cost. Furthermore, the gas heaters provide fast recovery of hot water when the current supply of hot water in the storage tank is depleted. The relatively high fuel and operating costs for natural gas and electric water heaters, however, are underperceived and underestimated by consumers. The hot water energy requirement for the average household in the United States is in the range of 5,000 kilowatt hours per year.
Wood fired water heating burners, stoves and furnaces offer the advantage of low operating costs for fuel and maintenance using intermediate level technology. However, presently available wood fired water heaters such as wood stoves retrofitted with water circulating coils have slower recovery of hot water in the storage tank. Other available units have almost no water storage or low operating efficiency. Furthermore, the requirements for efficient wood fired water heating present a design dilemma which has not been resolved in current domestic wood fired water heaters.
On the one hand, efficient combustion of the wood-type solid fuel requires a hot turbulent refractory combustion zone with storage of heat in the refractory material to provide adequate time and temperature level for complete combustion of the fuel. On the other hand, for overall efficiency of operation and energy usage, all possible energy from the fuel should be transferred to the hot water by the end of the firing cycle rather than be stored in the refractory material of the combustion zone. This design dilemma is not present in wood fired space heating burners and stoves which may operate continuously, but results from the short term on and off operating cycles of hot water heating with consequent loss of energy stored in the refractory material during each cycle. Efficiency of combustion is sacrificed in currently available wood fired domestic water heaters which are designed with the combustion chamber and combustion zone in a water wall or water jacket.